The present application claims priority to Japanese Application(s) No(s). P2001-114268 filed Apr. 12, 2001, which application(s) is/are incorporated herein by reference to the extent permitted by law.
The present invention relates to a semiconductor laser device of ridge waveguide type. More particularly, the present invention relates to a semiconductor laser device of ridge waveguide type which has a desirably controlled half-width value xcex8// of a far field pattern (FFP) in a direction horizontal to a hetero interface, exhibits good laser characteristics during high-output operation and merely requires a low driving voltage.
Semiconductor laser devices of ridge waveguide type, including those which are based on GaAs or InP for long wavelengths and a nitride based III-V group compound for short wavelengths, find use in a various application areas because they are easy to manufacture.
The semiconductor laser device of ridge waveguide type belongs to the category of index guided device. It has an upper portion of an upper cladding layer and a contact layer, both resembling a striped-shaped ridge. The ridge is formed such that an insulating film covers both sides of the ridge and the upper cladding layer extending sideward from the base of the ridge. This insulating film functions as a layer to constrict electric current and provides an effective refractive index difference in the lateral direction for mode control.
An explanation is given below, with reference to FIG. 11, of the structure of a related-art nitride based III-V group compound semiconductor laser device of ridge waveguide type which emits light with a wavelength of about 410 nm. This laser device is referred to as xe2x80x9cnitride based semiconductor laser devicexe2x80x9d hereinafter.
FIG. 11 shows a related-art nitride based semiconductor laser device of ridge waveguide type 10 has basically a stacked structure in which a plularity of layers are stacked on a sapphire substrate 12. The plularity of layers stacked on the sapphire substrate 12 are a laterally grown GaN layer 14, an n-GaN contact layer 16, an n-AlGaN cladding layer 18, an active layer 20, a p-AlGaN cladding layer 22, and a p-GaN contact layer 24.
In the stacked structure, the upper portion of the p-AlGaN cladding layer 22 and the p-GaN contact layer 24 are formed as a striped-shaped ridge 26. A mesa structure extending in the same direction as the ridge 26 is formed by the upper portion of the n-GaN contact layer 16, the n-AlGaN cladding layer 18, the active layer 20, and the remaining portion 22a of the p-AlGaN cladding layer 22.
The ridge 26 has a width (W) of about 1.7 xcexcm. The remaining portion 22a of the p-AlGaN cladding layer 22 which extends sideward from the base of the ridge 26 has a thickness (T) of about 0.17 xcexcm.
An insulating film 28 of SiO2 (about 2000 xc3x85 thick) is formed on both sides of the ridge 26, the side of the mesa structure above the p-AlGaN cladding layer 22 extending sideward from the base of the ridge 26, and the n-AlGaN contact layer 16.
On the insulating film 28 is formed a p-side electrode 30, which is in contact with the p-GaN contact layer 24 through a window in the insulating film 28. On the n-GaN contact layer 16 is formed an n-side electrode 32.
The nitride based semiconductor laser device of ridge waveguide type 10 mentioned above is considered as a highly efficient one because the insulating film 28 covering both sides of the ridge 26 is transparent to the emitted laser beam with little waveguide loss and the threshold current is small.
In the meantime, as its application areas expand, the nitride based semiconductor laser device of ridge waveguide type is required to have a higher kink level so that it maintains good characteristic property for light output vs. injected current throughout the region up to the high-output level. It is also required to have a larger half-width value xcex8// of a far field pattern (FFP) in a direction horizontal to the hetero interface.
For example, in the case where the nitride based semiconductor laser device is used as a light source of an optical pickup, it is required to have a larger half-width value xcex8//.
The results of the present inventors"" investigation revealed that the value of xcex8// is related closely with the difference (xcex94n) of effective refractive index of the ridge waveguide, as shown in FIG. 12. In order to obtain a larger value of xcex8//, it is necessary to have a larger value of xcex94n. Incidentally, the difference (xcex94n) of effective refractive index of the ridge waveguide is defined as neff1xe2x88x92neff2 or a difference between neff1 which is the effective refractive index of the ridge for the oscillation wavelength and neff2 which is the effective refractive index of the ridge""s side, as shown in FIG. 11. Closed and open circles in FIG. 12 denote the values obtained by experiments.
Unfortunately, any attempt to increase the value of xcex94n ends up with a narrow cutoff ridge width of high-order horizontal lateral mode. The cutoff ridge width of high-order horizontal lateral mode is defined as a ridge width which gives rise to no high-order horizontal lateral mode. When the ridge width is larger than the cutoff ridge width, the horizontal lateral mode tends to shift from the fundamental mode to the primary high-order mode at the time of laser oscillation.
When a hybrid mode consisting of the fundamental horizontal lateral mode and the high-order horizontal lateral mode occurs, a kink occurs in the light output-injected current characteristics, as shown in FIG. 13. The result is a deterioration in the laser characteristics at the time of high-output operation.
The foregoing holds true particular for the nitride based semiconductor laser device of ridge wave-guide type, which has a small value of xcex94n and a short oscillation wavelength and hence has a narrow cutoff ridge width of high-order horizontal lateral mode, as shown in FIG. 14. FIG. 14 is a graph showing the relation between the value of xcex94n and the cutoff ridge width in the case where the GaN layer has a refractive index of 2.504 and an oscillation wavelength (xcex) of 400 nm. xcex94n stands for the difference between the effective refractive index of the ridge and the effective refractive index of the ridge""s side. For example, if the value of xcex94n is 0.005 to 0.01, the ridge width should be reduced to about 1 xcexcm so that the ridge width is smaller than the cutoff ridge width.
As mentioned above, any attempt to increase the value of xcex94n, thereby increasing the value of xcex8//, ends up with a decreased cutoff ridge width, which leads to a deterioration in laser characteristics at the time of high-output operation. In other words, there is a tradeoff for ridge width between the value of xcex8// and the laser characteristics at the time of high-output operation.
Moreover, the nitride based semiconductor laser device of ridge waveguide type has found an increasing use in the area of portable machines. The one for this purpose is required to have a lower drive voltage. One way to reduce the drive voltage is to increase the ridge width so that the contact area between the contact layer and the p-side electrode is increased. However, this suffers the disadvantage that the ridge width exceeds the cutoff ridge width, resulting in a deterioration in the laser characteristics at the time of high-output operation. In other words, there is a trade-off for the ridge width between the reduced drive voltage and the improved laser characteristics at the time of high-output operation.
The foregoing indicates that reducing the ridge width, thereby improving the laser characteristics at the time of high-output operation, contradicts increasing the value of xcex8// and decreasing the drive voltage.
As mentioned above, the related-art nitride based semiconductor laser device poses several problems. That is, it does not permit the ridge width to be decreased appreciably in order to keep its drive voltage low. Also, it has a ridge width lager than its cutoff ridge width, which prevents the kink level from being raised to a desired high level in the light output-injected current characteristics. The result is that the value of xcex94n is small and the value of xcex8// is also small.
The foregoing is applicable not only to nitride based semiconductor laser devices but also to any semiconductor laser devices (such as GaAs and InP) of ridge waveguide type for longer wavelengths.
It is an object of the present invention to provide a semiconductor laser device of ridge waveguide type which has a low drive voltage, a large value of xcex8//, and a high kink level or good light output-injected current characteristics up to a high output range.
The present inventors carried out extensive investigations in search of a semiconductor laser device which has a large value of xcex94n and a large value of xcex8// and keeps good light output-injected current characteristics up to a high output range, without the necessity of reducing the ridge width to keep the drive voltage low. As the result, it was found that a difference in absorption coefficient occurs between the fundamental horizontal lateral mode and the primary horizontal lateral mode, as shown in FIG. 15, if stacked layers are sequentially formed on both sides of the ridge, the stacked layers consisting of an insulating film which does not absorb the laser beam appreciably, an insulating film which is substantially transparent to the light of the oscillation wavelength, and a film which absorbs the laser beam.
It was also found that the above-mentioned phenomenon can be utilized to increase the kink level as high as practically acceptable and to increase the value of xcex8// without reducing the ridge width.
In addition, the present inventors carried out a series of experiments on the combination of various insulating films and absorption films. As the result, it was found that the insulating films and absorption films in stacked form (each film having a thickness specified in the present invention) suppress the high-order lateral mode. These findings led to the present invention.
The present invention to achieve the above-mentioned object is directed to a semiconductor laser device of ridge waveguide type including: a ridge formed in an upper portion of at least an upper cladding layer, wherein a stacked film composed of an insulating film substantially transparent to the oscillation wavelength and an absorption film formed on the insulating film which absorbs the oscillation wavelength, is formed on both sides of the ridge and on the upper cladding layer extending sideward from the base of the ridge, an electrode film is electrically connected to the upper surface of the ridge through a window in the stacked film, and the insulating film and the absorption film have respective thicknesses such that the absorption coefficient of high-order horizontal lateral mode is larger than the absorption coefficient of fundamental horizontal lateral mode.
According to the present invention, the ridge may be in any form (in plan view) without specific restrictions. It may be in a stripe form, taper form, or flare form.
The fact that the insulating film and the absorption film have film thicknesses which are respectively established such that the absorption coefficient of high-order horizontal lateral mode is larger than the absorption coefficient of fundamental horizontal lateral mode suppresses the high-order horizontal lateral mode and increases the kink level in the high output region without the necessity of reducing the ridge width, and also increases the value of xcex94n and the value of xcex8//.
According to the present invention, the insulating film is not specifically restricted in its kind so long as it is transparent to the oscillation wavelength, and the absorption film is not specifically restricted in its kind so long as it absorbs the oscillation wavelength.
xe2x80x9cInsulating film substantially transparent to the oscillation wavelengthxe2x80x9d means a film whose absorption edge is shorter than the oscillation wavelength. xe2x80x9cAbsorption filmxe2x80x9d means a film whose absorption edge is longer than the oscillation wavelength.
According to the present invention, the insulating film may be any of an SiO2 film, Al2O3 film, AlN film, SiNx film, Ta2O5 film, and ZrO2 film, and the absorption film may be an Si film which is usually an amorphous Si film.
The insulating film such as SiO2 film, Si film, and ZrO2 film should preferably be formed by vapor deposition.
The semiconductor laser device of the present invention has a resonator structure of nitride based III-V group compound semiconductor layer formed on a substrate and also has an AlGaN cladding layer (as an upper cladding layer) whose upper portion is formed as the ridge. In this semiconductor laser device, the insulating film (SiO2 film) has a thickness of 200 xc3x85 to 800 xc3x85 and the absorption film (Si film) has a thickness of 50 xc3x85 and above.
The thickness of the Si film, which is 50 xc3x85 and above, has been established from the following simulation. The simulation was run with a model in which the insulating film consists of an SiO2 film with a constant thickness of 600 xc3x85 and an Si film with a varied thickness, as shown in FIG. 16A. Calculations by this simulation predicted the change in absorption coefficient of the fundamental horizontal lateral mode and the primary horizontal lateral mode, as shown in FIG. 16B. The curve (1) represents the absorption coefficient of fundamental horizontal lateral mode, and the curve (2) represents the absorption coefficient of primary horizontal lateral mode.
Since it is desirable that the absorption coefficient xcex1 of primary horizontal lateral mode be at least 10 cmxe2x88x921, the thickness of the Si film should be equal to or larger than 50 xc3x85, preferably equal to or larger than 200 xc3x85.
For desirable results, the thickness of the SiO2 film as an insulating film should be 400 xc3x85 to 800 xc3x85 and the thickness of the Si film as an absorption film should be 50 xc3x85 and above. For more desirable results, the thickness of the SiO2 film as an insulating film should be 400 xc3x85 to 800 xc3x85 and the thickness of the Si film as an absorption film should be 200 xc3x85 and above.
If the thickness of the SiO2 film exceeds 800 xc3x85, there will be no difference between the absorption coefficient of high-order horizontal lateral mode and the coefficient of fundamental horizontal lateral mode, which results in a small value of xcex94n. By contrast, if the thickness of the SiO2 film is not more than 400 xc3x85, the absorption coefficient of fundamental horizontal lateral mode is excessively small, which results in an increased threshold current.
The thickness of the ZrO2 film as an insulating film should be 200 xc3x85 to 1200 xc3x85 and the thickness of the Si film as an absorption film should be 50 xc3x85 and above. For desirable results, the thickness of the ZrO2 film as an insulating film should be 300 xc3x85 to 1100 xc3x85 and the thickness of the Si film as an absorption film should be 50 xc3x85 and above. For more desirable results, the thickness of the ZrO2 film as an insulating film should be 600 xc3x85 to 1100 xc3x85 and the thickness of the Si film as an absorption film should be 200 xc3x85 and above.
If the thickness of the ZrO2 film exceeds 1200 xc3x85, there will be no difference between the absorption coefficient of high-order horizontal lateral mode and the coefficient of fundamental horizontal lateral mode, which results in a small value of xcex94n. By contrast, if the thickness of the ZrO2 film is not more than 200 xc3x85, the absorption coefficient of fundamental horizontal lateral mode is excessively small, which results in an increased threshold current.
Furthermore, the insulating film may be replaced by any of an Al2O3 film (200 xc3x85 to 1000 xc3x85 thick), an SiNx film (200 xc3x85 to 1200 xc3x85 thick), an AlN film (200 xc3x85 to 1400 xc3x85 thick), a Ta2O5 film (200 xc3x85 to 1200 xc3x85 thick), and a ZrO2 film (200 xc3x85 to 1200 xc3x85 thick). The insulating film is combined with an Si film with a thickness of 50 xc3x85 and above as an absorption film to form the stacked film.
Alternatively, the stacked film may be a combination of a metal film as an absorption film and any of the following insulating films: an Al2O3 film (200 xc3x85 to 1000 xc3x85 thick), an SiNx film (200 xc3x85 to 1200 xc3x85 thick), an AlN film (200 xc3x85 to 1400 xc3x85 thick), a Ta2O5 film (200 xc3x85 to 1200 xc3x85 thick), and a ZrO2 film (200 xc3x85 to 1000 xc3x85 thick).
The stacked film may also be formed from an SiO2 film (100 xc3x85 to 800 xc3x85 thick) or a ZrO2 film (200 xc3x85 to 1000 xc3x85 thick) as an insulating film and a metal film as an absorption film. The metal film may be formed from Ni, Pt, or Au with a thickness of 10, 100, or 300 nm, respectively. The metal film may function as an electrode.